1,585 research outputs found

### Unconditional Pointer States from Conditional Master Equations

When part of the environment responsible for decoherence is used to extract
information about the decohering system, the preferred {\it pointer states}
remain unchanged. This conclusion -- reached for a specific class of models --
is investigated in a general setting of conditional master equations using
suitable generalizations of predictability sieve. We also find indications that
the einselected states are easiest to infer from the measurements carried out
on the environment.Comment: 4 pages, 3 .eps figures; final version to appear in Phys.Rev.Let

### Testing quantum adiabaticity with quench echo

Adiabaticity of quantum evolution is important in many settings. One example
is the adiabatic quantum computation. Nevertheless, up to now, there is no
effective method to test the adiabaticity of the evolution when the
eigenenergies of the driven Hamiltonian are not known. We propose a simple
method to check adiabaticity of a quantum process for an arbitrary quantum
system. We further propose a operational method for finding a uniformly
adiabatic quench scheme based on Kibble-Zurek mechanism for the case when the
initial and the final Hamiltonians are given. This method should help in
implementing adiabatic quantum computation.Comment: This is a new version. Some typos in the New Journal of Physics
version have been correcte

### A ring of BEC pools as a trap for persistent flow

Mott insulator - superfluid transition in a periodic lattice of Josephson
junctions can be driven by tunneling rate increase. Resulting winding numbers
$W$ of the condensate wavefunction decrease with increasing quench time in
accord with the Kibble-Zurek mechanism (KZM). However, in very slow quenches
Bose-Hubbard dynamics rearranges wavefunction phase so that its random walk
cools, $\bar{W^2}$ decreases and eventually the wavefunction becomes too cold
to overcome potential barriers separating different $W$. Thus, in contrast with
KZM, in very slow quenches $\bar{W^2}$ is set by random walk with "critical"
step size, independently of $\tau_Q$.Comment: Decompressed version to appear in Phys. Rev.

### Quench from Mott Insulator to Superfluid

We study a linear ramp of the nearest-neighbor tunneling rate in the
Bose-Hubbard model driving the system from the Mott insulator state into the
superfluid phase. We employ the truncated Wigner approximation to simulate
linear quenches of a uniform system in 1,2, and 3 dimensions, and in a harmonic
trap in 3 dimensions. In all these setups the excitation energy decays like one
over third root of the quench time. The -1/3 scaling arises from an
impulse-adiabatic approximation - a variant of the Kibble-Zurek mechanism -
describing a crossover from non-adiabatic to adiabatic evolution when the
system begins to keep pace with the increasing tunneling rate.Comment: 10 pages, 13 figures; version published in Phys. Rev.

### The decoherence criterion

The decoherence mechanism signals the limits beyond which the system dynamics
approaches the classical behavior. We show that in some cases decoherence may
also signal the limits beyond which the system dynamics has to be described by
quantum field theory, rather than by quantum mechanics.Comment: revised paper, in print on Mod. Phys. Lett.

### Soliton creation during a Bose-Einstein condensation

We use stochastic Gross-Pitaevskii equation to study dynamics of
Bose-Einstein condensation. We show that cooling into a Bose-Einstein
condensate (BEC) can create solitons with density given by the cooling rate and
by the critical exponents of the transition. Thus, counting solitons left in
its wake should allow one to determine the critical exponents z and nu for a
BEC phase transition. The same information can be extracted from two-point
correlation functions.Comment: 4 pages, 3 figures, improved version to appear in PRL: scalings
discussed more extensively, fitting scheme for determination of z and nu
critical exponents is explaine

### Decoherence, Chaos, and the Second Law

We investigate implications of decoherence for quantum systems which are
classically chaotic. We show that, in open systems, the rate of von Neumann
entropy production quickly reaches an asymptotic value which is: (i)
independent of the system-environment coupling, (ii) dictated by the dynamics
of the system, and (iii) dominated by the largest Lyapunov exponent. These
results shed a new light on the correspondence between quantum and classical
dynamics as well as on the origins of the ``arrow of time.''Comment: 13 Pages, 2 Figures available upon request, Preprint LA-UR-93-, The
new version contains the text, the previous one had only the Macros: sorry

### Probabilities from envariance?

Zurek claims to have derived Born's rule noncircularly in the context of an
ontological no-collapse interpretation of quantum states, without any "deus ex
machina imposition of the symptoms of classicality." After a brief review of
Zurek's derivation it is argued that this claim is exaggerated if not wholly
unjustified. In order to demonstrate that Born's rule arises noncircularly from
deterministically evolving quantum states, it is not sufficient to assume that
quantum states are somehow associated with probabilities and then prove that
these probabilities are given by Born's rule. One has to show how irreducible
probabilities can arise in the context of an ontological no-collapse
interpretation of quantum states. It is argued that the reason why all attempts
to do this have so far failed is that quantum states are fundamentally
algorithms for computing correlations between possible measurement outcomes,
rather than evolving ontological states.Comment: To appear in IJQI; 9 pages, LaTe

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